There has been growing tension between the airlines and the city.
The airlines say delays are down and there isn't a need to complete the next phase of the expansion -- just yet.
But the city was apparently set to move ahead with the $3 billion project.
So, Tuesday, two of the largest carriers took action.
As O'Hare starts to see a slight bump in air travel following the recession, the airlines who fly more than 80 percent of the customers don't want to get too ahead of demand.
Tuesday, United and American airlines filed a lawsuit seeking to prevent the City of Chicago from issuing bonds to pay -- in part -- for the second phase of the O'Hare modernization project.
The airlines state they have been willing to do their part but moving forward now will burden their customers and their abilities to grow.
The airlines contend that their leases allow them to approve of expansion projects:
"If the city proceeds as threatened without airline approval, there will be no way to unring the bell...The projects will move forward at exorbitant expense financed by massive, unauthorized bond offerings."
Last week, Mayor Richard M. Daley voiced plans to move ahead with phase two.
"We're very confident where we are," Daley said. "This is important because we're not building runways for the airlines today. We're building runways for passengers so there's no delays in bad weather or anything like that."
Phase two would reconfigure the runways, giving pilots more flexibility in challenging Chicago weather.
United and American want to see phase two completed, but they say they want to see it at a time when demand returns and profitability increases, saying:
"The commercial aviation industry, like the country as a whole, has suffered great losses over the last several years."
The City of Chicago issued a statement saying it continues to move forward and believes it has a legal right to do so, saying that the project is creating jobs and stimulating the economy, but it is not appropriate to comment further on the litigation at this time.